Such a process is for example known from patent application EP1574539A1. This document describes a process for making PET wherein PTA and EG are reacted in the presence of a catalyst system consisting of 15-150 ppm of Sb-compound and 40-160 ppm of Zn-compound as active catalyst components and 10-30 ppm of phosphoric acid as stabilizing component (ppm based on PET). Compared with a typical standard Sb-catalyst, this catalyst system is indicated not only to increase productivity in both melt-phase polycondensation and in a subsequent solid-phase polycondensation (SSP) step, but also to enhance optical properties of the PET; i.e. moulded product shows improved clarity and reduced greyish colour than standard Sb-based PET, which is generally ascribed to presence of Sb-catalyst residues. In order to obtain sufficiently fast reactions, the catalyst system is used at a concentration resulting in total metal content of the PET obtained of typically higher than 200 ppm.
Polyesters like PET are well-known in the art, and are widely applied in applications like textile and industrial fibres, films and sheets, and containers, especially bottles. Initial PET production employed dimethyl terephthalate (DMT) and ethylene glycol (also called monoethylene glycol; EG) as precursors, but nowadays production plants generally use purified terephthalic acid (PTA) and EG as raw materials, because of process economic reasons. In this case, first an oligomer or low molecular mass prepolymer is formed by esterification of PTA with a molar excess of EG to form diethyleneglycol terephthalate (also called bis hydroxyethyl terephthalate) and oligomers thereof (together referred to hereinafter as DGT), with water being the main by-product distilled off (step a)). This step is generally self-catalysed, but may be accelerated by adding catalyst. DGT is further subjected to polycondensation by transesterification reactions to form higher molecular mass polyester (step b)). In this step, DGT is heated to about 280° C. under high vacuum to carry out the melt-phase polycondensation with removal of EG liberated in the polycondensation reaction. Because the transesterification is a slow reaction, the polycondensation step is generally catalysed. This catalyst can be added in step b), but it can also already be included in step a). The melt is discharged and made into pellets after it reaches a desired molecular mass, reflected by intrinsic viscosity (IV) values. Commercial-scale PET production is generally based on a continuous PTA system employing several reactors in series, as described for example by S. M. Aharoni in “Handbook of Thermoplastic Polyesters”, vol. 1, chapter 2, Editor S. Fakirov, Wiley-VCH, 2002; and by V. B. Gupta and Z. Bashir in “Handbook of Thermoplastic Polyesters”, vol. 1, chapter 7, Editor S. Fakirov, Wiley-VCH, 2002. Typically, such a system includes a vessel in which EG, PTA, catalyst and additives are mixed to form a paste; one or more esterification reactors; one or more pre-polycondensation reactors, followed by a high-vacuum, finisher reactor for the final stages of polycondensation. The polyester formed may be extruded into strands, quenched under water and cut to form pellets or chips. PET used in film and fibre applications typically has an IV in the range of 0.55 to 0.65 dL/g; PET films and fibres can also be produced directly by extruding the melt from the polycondensation reactor. For PET bottle grade resin, polymers with IV in the range of 0.75 to 0.85 dL/g, and having low residual acetaldehyde are generally required. In this case, a split process is used to attain this IV value while attaining a low amount of acetaldehyde. The general practice is to make polymer chips with an intermediate IV of about 0.63 dL/g by melt-polycondensation, and then increase the IV by subsequent solid-state polycondensation (SSP). This split procedure allows production of a high IV resin with minimal quantities of acetaldehyde, which is a degradation by-product that affects the taste of beverages packed in PET bottles. Diethylene glycol (DEG) is a diol generated from ethylene glycol via a side-reaction and is also incorporated in the PET chain. Presence of DEG as comonomer reduces the glass transition and melting temperature of the PET, but too high levels are undesirable. The melt-phase and SSP technology is described for example in Encyclopaedia of Polymer Science and Engineering, 2nd ed, volume 12, John Wiley and Sons, New York (1988), and in “Handbook of Thermoplastic Polyesters”, vol. 1, chapter 7, Editor S. Fakirov, Wiley-VCH, 2002.
In the literature many different metal-based catalysts have been reported to be suitable for polyester and especially for PET production. The activity as catalyst for the polycondensation reaction of various metals on a molar basis follows the trend Ti>Sn>Sb>Mn>Zn>Pb; see for example F. Fourne, “Synthetic fibres”, Hanser Verlag (1999), p. 67 ff. In selecting a catalyst, however, besides activity towards (trans-)esterification and polycondensation also its effect on undesirable side-reactions needs to be considered.
Relevant side-reactions during PET production include: (1) formation of acetaldehyde, which may affect taste of products packed in PET containers; (2) formation of COOH-endgroups, which affect hydrolytic and thermal stability of the polyester, and an unbalanced number of endgroups may limit molar mass increase; (3) formation of vinylester endgroups, which are not active towards polycondensation; (4) formation of diethyleneglycol (DEG); which is incorporated as comonomer; and (5) formation of chromophores; causing e.g. yellowing.
Although titanium is in principle the most active metal, the catalysts currently employed in more than 90% of industrial PET production are based on antimony (Sb) as they give the best balance in activity and polymer performance. Ti- and Zn-based systems generally result in e.g. too slow solid-state reactions, unacceptable yellowing, and/or high acetaldehyde generation. Typically, about 200-300 ppm Sb (mostly added as antimony acetates, oxides, or glycolates; ppm metal based on PET) is used to provide sufficiently fast reactions. Sb-based PET also shows some yellow discolouration, but this can be effectively masked by adding colour correction agents, like a Co-compound. A further disadvantage of using antimony-based catalysts is the slight greyish colour of PET that is reported to result from precipitation of antimony metal particles. In addition, antimony is rather expensive and subject to some environmental concerns. Increasing the amount of Sb present in PET to above about 300 ppm presents thus no advantages.
Various publications have addressed mixed metal catalysts systems for PET that enable lower amounts of antimony to be used, e.g. by combining Sb with a second or third metal compound to result in some synergistic effect. For example, U.S. Pat. No. 5,008,230 and U.S. Pat. No. 5,166,311 describe a tri-component catalyst based on antimony, 5-60 ppm of cobalt and/or zinc, and 10-150 ppm of zinc, magnesium, manganese or calcium. The catalyst would allow reducing melt-polycondensation times by at least one-third, compared to the conventional antimony catalyst. Other patent publications covering Sb—Zn catalyst compositions include U.S. Pat. No. 5,162,488, and EP0399742.
U.S. Pat. No. 5,623,047 states that the optical appearance of PET made with the PTA process can be improved by introducing alkali metal acetate as third component besides antimony and at least one of cobalt, magnesium, zinc, manganese and lead.
U.S. Pat. No. 5,608,032 discloses a catalyst system that contains 10-1000 ppm Sb, 10-500 ppm of at least one of Co, Mg, Zn, Mn, Ca and Pb, and 10-500 ppm of a P-compound. In U.S. Pat. No. 4,356,299 PET of relatively low IV and slightly yellow colour is made with a combination of 40-300 ppm Sb and 2-18 ppm Ti-compound.
A mixed metal catalyst containing 50-300 ppm Sb, 25-100 ppm Mn, 10-100 ppm Co and 20-60 ppm Ti from titanium alkoxide was used in U.S. Pat. No. 4,010,145 to make PET via DMT route.
U.S. Pat. No. 5,017,680 discloses a catalyst system for making PET via the DMT process, which contains 150-350 ppm Sb, 25-110 ppm Mn, Zn or Ca, 10-100 ppm Co, and 10-100 ppm Ti preferably as a complex of titanium alkoxide with an alkali or alkaline earth metal. In the experiments catalyst compositions are used that contain 200-220 ppm Sb, 50-60 ppm Mn, 18-21 ppm Ti, and 40-80 ppm Co as metal components; resulting in a polyester with a metal content of over 310 ppm.
In JP2000226446 bottle-grade PET is made with a catalyst system consisting of 12-207 ppm Sb, 2-300 ppm of at least one of Mg, Ca, Co, Mn, or Zn, 0-20 ppm Ti, 0-50 ppm Ge, and a basic nitrogen compound as essential components. The experiments apply Sb—Mg—Ti—N catalyst compositions. Use of a nitrogen compound in polyesters may have negative effects on colour and on organo-leptic properties.
WO2008/008813A2 describes a catalyst system that is especially suited to promote the SSP step in a process for making polyester like PET, which catalyst system comprises as active components 1) a coordination catalyst component selected from Ti, Ge, Sb and Al, 2) a strong acid component of certain pKa value, and 3) optionally a supplemental catalyst component selected from Co, Mn and Zn. Catalyst compositions with two or more metal components actually disclosed contain 7 ppm Ti/30 ppm Co; 240-250 ppm Sb/10-25 ppm Co; and 30 ppm Sb/150 ppm Ge/10 ppm Co; all three further containing 200-1000 ppm of strong acid.
There is, however, still a general need in the industry for an economical process for making PET from EG, PTA and optionally comonomer, using a mixed metal catalyst system that results in PET showing a good balance of mechanical and optical properties, and containing reduced amounts of metal residues, especially less heavy metals.